feat(settings): add default subscriptions for new users

- add default subscriptions to admin settings

- auto-assign subscriptions on register and admin user creation

- add validation/tests and align settings UI with subscription selector patterns

backend/internal/server/middleware/admin_auth_test.go

@@ -19,7 +19,7 @@ func TestAdminAuthJWTValidatesTokenVersion(t *testing.T) {
 	gin.SetMode(gin.TestMode)
 
 	cfg := &config.Config{JWT: config.JWTConfig{Secret: "test-secret", ExpireHour: 1}}
-	authService := service.NewAuthService(nil, nil, nil, cfg, nil, nil, nil, nil, nil)
+	authService := service.NewAuthService(nil, nil, nil, cfg, nil, nil, nil, nil, nil, nil)
 
 	admin := &service.User{
 		ID:           1,